There's no guarantee anymore. It's not that there are wall-to-wall, elbow-to-elbow people in the parks every single second of every day -- there are pockets of time in which the crowds are slightly lower. However, people try to find those pockets of time so they can go with lighter crowds, and the crowd dynamics shift. The formerly-less-crowded times suddenly get more crowded, and the formerly-more-crowded times get less crowded!
Everyone pretty much agrees that there is almost no slow time anymore, especially during popular seasons such as Halloween Time and the Holidays. Rain or any kind of extreme weather (a bad heat wave, heavy wind, etc.) would keep the crowds down a bit.
In 2015, there is a good chance that Mickey's Halloween Party will begin on Friday, September 25th, and there would
probably be another party on Tues., 9/29. So the start of the MHP will attract crowds, and your 9/24-9/30 time frame includes a weekend, which will draw in crowds too.
Your October 14-20 dates (which also include a weekend) would probably fall right in the middle of or right around the UEA time frame -- the time when "Utah Escapes to Anaheim," as it is jokingly called, due to an educators' conference, and that means a lot of people in the parks.
It comes down to picking the lesser of two evils, I suppose. It depends on when UEA is happening in 2015. If it is over by mid-October, then October might be the better time to go (before Halloween week). If UEA is happening right during your October dates, then I
guess the September time frame would be better -- but it won't be empty or uncrowded in the parks.
Keep in mind, also, that next year is the 60th anniversary of
Disneyland, and it is highly possible that there could be random special events held on certain days/nights throughout the year in celebration of that. There could be pop-up events, hard ticket party-types of events (aside from the MHP), AP-only events, etc. No one knows exactly what is in store for the events of the 60th. Anything could happen, and any of those scenarios could affect crowds as well.